1. Field of the Invention
The present invention concerns a tomosynthetic image reconstruction method, in particular such a method suitable for mammography, in which a tomosynthetic 3D x-ray image is assembled from a number of digital 2D projection images acquired from various projection angles. Moreover, the invention concerns a diagnostic apparatus operating with such a method.
2. Description of the Prior Art
Mammography is an x-ray examination of the female breast, with the goal of detecting tumors in as early a stage as possible. Through steady improvement in mammography methods it is intended to generate x-ray images with good clarity in order to differentiate benign from malignant variations, and to reduce the number of incorrect findings (i.e. the number of suspicious findings that are caused by non-malignant variations) and the number of undetected malignant tumors. In conventional x-ray mammography, a two-dimensional single image of the compressed breast is generated in a single projection direction. Since the tissue layers lying atop one another in the direction of the x-ray beam are superimposed in such a projection, strongly absorbent benign structures can overlap a malignant tumor and complicate the ability to detect such a tumor.
In order to avoid this problem, mammography methods (known as tomosynthesis) are known in which 2D projection images or 2D projection data of the female breast are acquired in a number of different projection directions with a digital x-ray detector. Using image reconstruction methods, a three-dimensional image data set composed of a number of slices images, which respectively reproduce a slice of the breast oriented parallel to the acquisition surface of the x-ray detector, can be generated from 2D projection images (i.e. from the image data belonging to these 2D projection images) acquired from different projection angles. Such an image data set acquired by such reconstruction is designated in the following as a tomosynthetic 3D x-ray image. Tissue structures that lie deeper (as viewed in the propagation direction of the x-ray beam) can be better detected with this technique.
In the generation of the individual 2D projection images, the total dose allowed for a mammography exposure cannot be exceeded, such that these individual 2D projection images must be acquired with a dose that amounts to only a fraction of the permissible total dose (depending on the number of the 2D projection images used for the reconstruction), such that the individual 2D projection images are very noisy. In order to avoid transferring this high image noise to the tomosynthetic 3D x-ray image in the reconstruction, it is possible in principle to subject the individual 2D projection images to a noise-reducing image processing method before the reconstruction. Such a noise-reduction of the 2D projection images, however, has the result that the micro-calcifications necessary for a correct finding and indicative of an early tumor stage either disappear or, in the event that these exist in a cluster, merge with one another and appear as a larger benign calcium deposit in the reconstructed slice. This can lead to a misdiagnosis.